A modern turbojet gas turbine engine typically includes a bypass air fan section, and a separate central engine core consisting of a compressor, at least one combustor, and a turbine. The bypass air fan section, situated at an axially forward end of the engine, comprises a rotatable hub, an array of fan blades projecting radially from the hub and a fan casing encircling the blade array. In operation, the fan section forces a major portion of its received air around the central engine core, and the balance of the air into a flow passage leading to an axial compressor. The portion of the air passing through the compressor is pressurized, then directed into the combustor. Fuel is continuously injected into the combustor together with the compressed air. The mixture of incoming fuel and air is ignited to create combustion gases that enter a combustion section of the rotatably driven turbine. As a result, high temperature, high pressure combustion gases expand rapidly over rotating blades and static vanes of the turbine. Since the turbine is connected to the compressor via a shaft, the combustion gases that drive the turbine also drive the compressor to maintain continuous operation of the engine.
The turbine vanes in the combustion section of a gas turbine engine are fixed in place within a so-called “hot section” of the engine, and may be subject to an environment having temperatures that range up to 2,000 degrees Celsius. Although the base metals of such vanes are generally formed of super alloys including cobalt or nickel, the working surfaces of the vanes are typically coated with ceramic to assure longevity under harsh operating conditions. The finished surfaces of the ceramic coatings must be extremely smooth and micro-crack free to perform at optimal levels.
Ceramic coatings are normally applied to outer base metal surfaces of the components via plasma spray techniques that are well known in the art. Machining operations required to smooth out and hence finish the ceramic surfaces have involved using mixtures of abrasive stone particles in water baths. Such mixtures are vibrated, often within bowls containing pluralities of the ceramic coated components desired to be finished. The stone particles are available in a variety of sizes and shapes, some having average diameters on the order of up to 0.5 inch, depending on the desired smoothness and length of vibratory exposure time.
The approach involves a relatively messy slurry bath, to the extent that water and/or other liquid media are used for greatest effectiveness in polishing exterior surfaces of the coated ceramic. Significant cleanup is required between batches, as well as replacement of the abrasive stone particles as they become reduced in size due to wear. A further disadvantage has been an inability to achieve surface finishes having roughness averages of less than 150 microinches. Thus, in some instances, the polished surfaces may not become as smooth as desired.
To better achieve current ceramic surface smoothness demanded by the turbojet gas turbine industry to produce even more reliable high-performance turbine engines, it is therefore desirable to provide improved machine processing methods having lower costs and shorter time requirements.